1. Field of the Invention
The present invention relates to a fuel assembly which is used in a pressurized water reactor.
2. Description of Related Art
A conventional fuel assembly used in a pressurized water reactor, which includes, a lower nozzle disposed on a lower core plate of the pressurized water reactor, an upper nozzle having a hold-down spring for holding down the lower nozzle against the lower core plate, a plurality of control rod guide thimbles for guiding control rods extending through the upper nozzle toward the lower core plate, a plurality of supporting grids mounted on the control rod guide thimbles, and a number of fuel rods held by the supporting grids in parallel with the control rod guide thimbles, is disclosed in a pamphlet published by FRAMATOME Co. entitled "SNE97-La Coruna-Nov. 5-7, 1997".
In a fuel assembly such as mentioned above, when the control rods, detached from the control rod driving unit upon emergency shutdown of the nuclear reactor, fall within the control rod guide thimbles without being decelerated, the upper nozzle of the fuel assembly may be subjected to an excessively large impact force. Consequently, dashpots of a thin-tube configuration are formed within the control rod guide thimbles so that the speed at which the control rods fall within the control rod guide thimbles is decreased to thereby dampen the impact force applied to the upper nozzle.
However, in a conventional fuel assembly such as described above, the dashpot has a length ranging from 0.16 L to 0.18 L, wherein L represents the entire length of a control rod guide thimble. Consequently, flexural deformation may take place in the dashpot under compression loads acting in the axial direction of the control rod guide thimble, giving rise to problems in that insertability of the control rod may be impaired due to the flexural deformation of the dashpot.